1. Field of the Invention
The present invention relates to an amplifier and a reception device and transmission device using the same. More particularly, the present invention relates to an amplifying device with a distortion compensation function, and a reception device and transmission device using the same.
2. Description of the Background Art
Recently, rapid progress has been made in improving mobile communication terminals. Particularly, various functions as well as a communication function have been incorporated into a mobile communication terminal. As the mobile communication terminal is increasingly becoming widespread, there is a keen demand for a reduction in the power consumption thereof. In a wireless circuit of the mobile communication terminal, an amplifier circuit, a mixer circuit and the like are predominant factors for the power consumption. Therefore, it is an important challenge for the amplifier circuit to attain a predetermined function while suppressing the power consumption.
In the amplifier circuit, for example, when a signal is input thereinto, inter-modulation distortion occurs due to signals of two bands included in the received signal. Particularly, third-order inter-modulation distortion (hereinafter referred to as IM3) occurs in the vicinity of the received signal, affecting the received signal. Also, when a transmission signal is input to the amplifier circuit, IM3 occurs due to an influence of the bandwidth of the transmission signal, affecting the transmission signal. Thus, it is also an important challenge to suppress the occurrence of IM3 in the amplifier circuit and the like.
It is conventionally known that the occurrence of IM3 can be suppressed by increasing the power consumption of the amplifier circuit. Although the occurrence of IM3 is suppressed, power consumption increases. Thus, the two challenges are not simultaneously solved. Therefore, various methods for suppressing the occurrence of IM3 without an increase in power consumption have been proposed.
As a first exemplary method of reducing IM3, a method of using a feedback-type amplifier circuit has been proposed. Patent publications 1 and 2 describe exemplary feedback-type amplifier circuits.
FIG. 32 is a diagram illustrating a feedback-type amplifier circuit described in Patent Publication 1 (Japanese PCT National Phase Laid-Open Patent Publication No. 2002-536859). In FIG. 32, a signal generator 1501 generates a signal, which is in turn input to an amplifier circuit 1502. A first transistor 1521 and a second transistor 1522 amplify an input signal. During signal amplification, an IM3 component occurs in the first transistor 1521 and the second transistor 1522. A shunt reactive feedback network 1523 includes a capacitor. The amplified signal and the occurring IM3 component are input to the shunt reactive feedback network 1523. The shunt reactive feedback network 1523 changes the phases of the amplified signal and the IM3 component, and feeds the resultant signal and IM3 component back to the base of the first transistor 1521. As a result, the IM3 component occurring during amplification is canceled with the IM3 component whose phase is changed during feedback.
FIG. 33 is a diagram illustrating a feedback-type amplifier circuit described in Patent Publication 2 (Japanese Laid-Open Patent Publication No. 2003-289226). In FIG. 33, an amplifier circuit 1602 amplifies a signal input through an internal terminal 1601. During signal amplification, an IM3 component occurs in the amplifier circuit 1602. A feedback circuit 1603 feeds the amplified signal back to the base of a first transistor 1621. During feedback of the amplified signal, the phases of the amplified signal and the IM3 component are changed by a first capacitor 1631. As a result, the IM3 component occurring in the amplifier circuit 1602 is canceled with the IM3 component whose phase is changed during feedback, thereby suppressing IM3.
As a second method of reducing IM3, a predistortion-type amplifier circuit has been proposed. The predistortion-type amplifier circuit comprises a noise generation section (first stage) and an amplification section (second stage). The noise generation section and the amplification section are connected in cascade.
FIG. 34 is a diagram illustrating the first stage circuit of a predistortion-type amplifier circuit described in Patent Publication 3 (Japanese Patent No. 3405401). In a FET 1701, noise occurs which has the same frequency band as that of an IM3 component occurring in the second stage during signal amplification. A first inductor 1702 changes the phase of the noise occurring in the FET 1701. A gate control voltage 1703 controls the amplitude of the noise occurring in the FET 1701. In this manner, the first stage generates a noise component having the same frequency band, the same amplitude, and the reverse phase with respect to IM3 which is expected to occur in the second stage during signal amplification. As a result, the noise component occurring in the first stage is canceled with the IM3 component occurring in the second stage during signal amplification.
However, the above-described methods have the following problems. In the first method, the phase of the IM3 component is changed, and a portion of the amplified signal and the IM3 component is fed back. In the first method, the IM3 component occurring in the amplifier circuit is canceled with the fed-back IM3 component, thereby suppressing IM3. However, the level of the input signal is reduced due to the feedback of the amplified signal. Therefore, the amplifier circuit cannot obtain a desired amplification level. Therefore, the amplified signal to be fed back needs to be limited, so that there is a limitation on suppression of IM3.
In the second method, the IM3 component which is generated on the first stage is canceled with the IM3 component occurring in the amplifier on the second stage, thereby suppressing IM3. However, when the IM3 component is generated on the first stage, white noise occurs in addition to the IM3 component. When the second method is applied to a transmission device, the occurrence of white noise leads to the following problem. The transmission device emits white noise in a frequency band which is different from that used by the transmission device. Therefore, a means for removing the white noise is additionally required, leading to a complicated device structure. In addition, there is a limit of removal of the white noise. When the second method is applied to a reception device, the occurrence of white noise leads to the following problem. A desired signal is not amplified on the first stage, and therefore, the level of the white noise becomes higher than the level of the IM3 component to a non-negligible extent. Therefore, a means for removing the white noise is additionally required, leading to a complicated device structure.